。
My sister has been studying in her room since this morning.
Literal
Younger-sister [topic-は] morning [from-から] all-along own [possessive-の] room [at-で] study [object-を] is-doing.
ずっと ('all along, the entire time') is an adverb that emphasizes the unbroken duration of an action. Combined with ~から ('since [a time]') and the ~ている progressive, it paints a picture of sustained activity from a starting point up to the present. The polite ~ています form keeps the register suitable for casual conversation with someone outside the family.